Removal of dust in urea finishing

ABSTRACT

Disclosed is a method for the removal of urea dust from the off-gas of a finishing section of a urea production plant. the method comprises subjecting the off-gas to quenching with water so as to produce quenched off-gas, and subjecting the quenched off-gas to scrubbing using at least one venturi scrubber. As a result, a lower pressure drop over the scrubber is attained, and a more efficient growth of urea particles, facilitating the removal thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a divisional of application Ser. No. 14/902,831(allowed) having an international filing date of 4 Jul. 2014, which isthe national phase of PCT application PCT/NL2014/050445 having aninternational filing date of 4 Jul. 2014, which claims benefit ofEuropean patent application No. 13175399.8 filed 5 Jul. 2013. Thecontents of the above patent applications are incorporated by referenceherein in their entirety.

The invention is in the field of urea production, and pertains to theremoval of urea dust from the off-gas associated with the production ofsolid urea particles (urea finishing). Particularly, the inventionpertains to the reduction of the emission of urea dust occurring fromsuch a urea plant finishing section. The invention also pertains to aurea production plant, and to revamping an existing urea productionplant.

BACKGROUND OF THE INVENTION

Urea is produced from ammonia and carbon dioxide. Today's ureaproduction involves relatively clean processes, particularly low in theemission of urea dust and ammonia. However, besides the chemicalsynthesis of urea, the production of urea on a commercial scale requiresthat the urea be presented in a suitable solid, particulate form. Tothis end, urea production involves a finishing step in which a urea meltis brought into the desired particulate form, generally involving anyone of prilling, granulation, and pelletizing.

Prilling used to be the most common method, in which the urea melt isdistributed, as droplets, in a prilling tower and whereby the dropletssolidify as they fall down. However, the end-product is often desired tohave a larger diameter and higher crushing strength than the oneresulting from the prilling technique. These drawbacks led to thedevelopment of the fluidized bed granulation technique, where the ureamelt is sprayed on granules that grow in size as the process continues.Prior to the injection in the granulator, formaldehyde is added toprevent caking and to increase the strength of the end-product.

In order to remove the energy released during crystallization, largeamounts of cooling air are fed to the granulation unit. The air thatleaves the finishing section contains, inter alia, urea dust. With aview to increased demand for urea production, and increasing legal andenvironmental requirements as to reduce the level of emissions, it isdesired that the urea dust is removed, and according to ever increasingstandards.

Over the past several decades the control of air pollution has become apriority concern of society. Many countries have developed highlyelaborate regulatory programs aimed at requiring factories, and othermajor sources of air pollution, to install the best available controltechnology (BACT) for removing contaminants from gaseous effluentstreams released into the atmosphere. The standards for air pollutioncontrol are becoming increasingly stringent, so that there is a constantdemand for ever more effective pollution control technologies. Inaddition, the operating costs of running pollution control equipment canbe substantial, and so there is also a constant demand for moreefficient technologies.

The removal of urea dust is challenging per se, since the amounts ofoff-gas (mainly air) are enormous, whilst the concentration of urea dustis low. A typical airstream is of the order of 750 000 Nm³/h. A typicalconcentration of urea dust therein is about 2 wt. %. Further, part ofthe urea dust is of a submicron size. Satisfying current standardsimplies the need to remove a major part of this submicron dust.

A further problem is that the large amounts of air needed in ureafinishing, results in this part of the production process being arelatively costly effort due to the need for very large extractor fanshaving a large electricity consumption. Particularly, when the air issubjected to scrubbing in order to reduce the emission of urea dust, andspecifically a major part of the submicron dust, into the atmosphere, arelatively large amount of energy is simply lost in the process, as aresult of the inevitable pressure drop in the scrubbing device.

One well known type of device for removing contaminants from a gaseouseffluent stream is a venturi scrubber. Venturi scrubbers are generallyrecognized as having the highest fine particle collection efficiency ofavailable scrubbing devices. In a venturi scrubber the effluent gas isforced or drawn through a venturi tube having a narrow “throat” portion.As the air moves through the throat it is accelerated to a highvelocity. A scrubbing liquid in the form of droplets, typically ofwater, is added to the venturi, usually at the throat, and enters thegas flow. The water droplets used are generally many orders of magnitudelarger than the contaminant particles to be collected and, as aconsequence, accelerate at a different rate through the venturi. Thedifferential acceleration causes interactions between the water dropletsand the contaminant particles, such that the contaminant particles arecollected by the water droplets. The collection mechanisms involve,primarily, collisions between the particles and the droplets anddiffusion of particles to the surface of the droplets. In either case,the particles are captured by the droplets. Depending on the size of thecontaminant particles, one or the other of these mechanisms maypredominate, with diffusion being the predominant collection mechanismfor very small particles, and collision or interception being thepredominant mechanism for larger particles. A venturi scrubber can alsobe efficient at collecting highly soluble gaseous compounds bydiffusion. A detailed description of these scrubbing mechanisms isdiscussed in Chapter 9 of Air Pollution Control Theory, M. Crawford,(McGraw-Hill 1976).

One of the main characteristics of this type of scrubber, is that itcauses a larger pressure drop than other scrubbers, the estimatedpressure drop required to reach the desired high collection efficiencybeing about 100 mbar. yet, in view of its suitability for the removal ofsubmicron particles (such as urea dust), it would be desired to make useof a venturi scrubber. It will be understood that using a venturi-typescrubbing device presents a further desire to reduce the inevitable lossof energy associated therewith.

Some background references refer to the use of venturi scrubbing in ureafinishing.

FR 2 600 553 relates to removing dust from gases, such as form from ureaprilling. The method as described includes subjecting the gas toprewashing, by spraying a liquid into the gas stream, prior to venturiscrubbing. The purpose of the pre-washing step is that no additionalscrubbing liquid is added, which would lead to a low pressure drop.I.e., the washing liquid is applied in such a way as to produce dropletsthat are of a sufficiently large size to wash out small particles.

EP 514 902 relates to a method for the removal of urea dust from theoff-gas of a finishing section of a urea production plant. Water isadded to act with a venturi scrubber, flowing down by gravity along thewalls of the venturi as a film. The gas flowing upward is atomizing thefilm thereby forming a scrubbing liquid, i.e. with the purpose to formliquid droplets that interact with ammonia, and optionally urea dust, tobe removed.

In fact, most venturi scrubbers in use today are “self-atomizing”, i.e.,the droplets are formed by allowing a liquid to flow into the throat ofthe venturi where it is atomized by the gas flow. While very simple toimplement, this method is not able to produce droplets of very smallmedian diameter.

The primary methods utilized in improving the collection efficiency of aventuri scrubber have been to decrease the size of the throat or toincrease the overall rate at which gas flows through the system. Both ofthese methods increase the differential velocities between thecontaminant particles and liquid droplets as they pass through thethroat of the venturi. This causes more interactions between particlesand droplets to occur, thereby improving contaminant removal. However,increasing the collection efficiency in this manner comes at a cost ofsignificantly higher energy input into the system, thereby resulting inhigher operating costs. The extra energy is expended due either to theincreased overall flow resistance attributable to the reduced throatdiameter, or to the increased overall flow rate through the venturi. Ineither case, the pressure drop across the venturi is increased andgreater pumping capacity is required. Accordingly, heretofore, effortsto increase the fine particle collection efficiency of a venturiscrubber have involved substantial increased energy input into thesystem.

Of particular concern to those in the field of air pollution control isthe collection of “optically active” particles. As used herein, the term“optically active particles” should be understood to mean particleshaving a diameter in the range of approximately 0.1 to 1.0 microns. Inan effort to control these particles, the EPA has recently reduced the“PM 2.5 standards” for the emissions of particles less than 2.5 microns.These and smaller particles are difficult to collect in conventionalventuri scrubbers due to their small size. Nonetheless, particles inthis size range are currently responsible for the measured emissions.

What is desired is an apparatus and method that permits the efficientand economical scrubbing of fine particles from a large gas flow using acleansing liquid in a venturi scrubber. Specific needs include reducedscrubbing liquid pumping requirements, lower pressure drop across theventuri, improved scrubber performance, and better control of thepressure drop across the venturi scrubber.

It is now desired to provide a method for treating the off-gas of a ureafinishing section in such a way as to effectively remove urea dust. Itis further desired to provide a method by which this removal isimproved. And, moreover, it is desired to achieve this in a process ofimproved energy efficiency.

Still another object of the present invention is to provide an airpollution control system which is capable of compensating for variationsin the flow through the system.

SUMMARY OF THE INVENTION

In order to better address one or more of the foregoing desires, theinvention, in one aspect, presents a method for the removal of urea dustfrom the off-gas of a finishing section of a urea production plant, themethod comprising subjecting the off-gas to quenching with water,particularly so as to produce quenched off-gas having a temperaturebelow about 45° C., and subjecting the quenched off-gas to scrubbingusing at least one venturi scrubber.

In another aspect, the invention pertains to a finishing equipment for aurea plant, said finishing equipment comprising a urea finishing devicecomprising an inlet for liquid urea, an inlet for cooling gas, acollector for solid urea, an outlet for off-gas and at least one venturiscrubber, wherein said outlet for off-gas is in fluid communication(e.g. via a gas flow line) with the venturi scrubber, and wherein aquenching system, such as a spray-quencher, is installed between theurea finishing device and the venturi scrubber.

In a yet another aspect, the invention provides a urea plant comprisinga synthesis and recovery section (A); said section being in fluidcommunication with an evaporation section (B), said evaporation sectionbeing in fluid communication with a finishing section (C) and having agas flow line to a condensation section (E); said finishing section (C)having a gas flow line to a dust scrubbing section (D), wherein the dustscrubbing section comprises at least one venturi scrubber (F), andwherein a quenching system (G) is installed between the finishingsection (C) and the venturi scrubber (F), said quenching system being,in fluid communication with the gas flow line between the finishingsection (C) and the dust scrubbing section (D).

In a still further aspect, the invention is a method of modifying anexisting urea plant, said plant comprising a synthesis and recoverysection (A); said section being in fluid communication with anevaporation section (B), said evaporation section being in fluidcommunication with a finishing section (C) and having a gas flow line toa condensation section (E); said finishing section (C) having a gas flowline to a dust scrubbing section (D), wherein the dust scrubbing section(D) is provided with at least one venturi scrubber, and wherein themethod comprises installing a quenching system (G) between the finishingsection (C) and the venturi scrubber (F), said quenching system being,in fluid communication with the gas flow line between the finishingsection (C) and the dust scrubbing section (D).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a block diagram of a urea plant having a finishingsection according to the invention.

FIG. 2 shows a schematic drawing of a dust scrubbing system used in thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

In a broad sense, the invention is based on the judicious insight toemploy quenching of the off-gas of a urea finishing section, incombination with the use of at least one venturi scrubber. Surprisingly,the quenching of the off-gas not only has advantageous effects on theconservation of energy, but also aids in a more efficient removal ofurea dust.

It will be understood that liquids used for washing a gas stream, due tothe different purpose of the liquid, are not applied in such a way as toinduce quenching of the gas. Quenching, as is applied in the presentinvention, has the purpose of cooling down the gas, preferably to atemperature below about 45° C. and creating a liquid saturation nearequilibrium. Preferably, the liquid is sprayed in such a way andconsistency that liquid droplets are formed that are so small that thedroplets evaporate quickly, and a liquid saturation in the vapour nearequilibrium is reached within a short time.

Preferably the quenching stream has a temperature of below 45° C., morepreferably below 40° C., most preferably below 35° C. The typical airtemperature of the off-gas exiting a finishing section of a urea plant,such as in fluid bed granulation, is about 110° C. After quenching, thetemperature is preferably below 45° C. Accordingly, the temperature ofthe gas stream is lowered by typically more than 50° C., preferably morethan 60° C., and most preferably more than 65° C.

Where, in this description, it is spoken of “fluid communication”, thisrefers to any connection between a first part or section of a plant anda second part or section of a plant via which fluids, notably liquids,can flow from the first part of the plant to the second part of theplant. Such fluid communication is typically provided by piping systems,hoses, or other devices well-known to the skilled person for thetransportation of fluids.

Where in this description it is spoken of “gas flow lines” this refersto any connection between a first part or section of a plant and asecond part or section of a plant via which gas or vapours, notablyaqueous vapours, can flow from the first part of the plant to the secondpart of the plant. Such gas flow lines typically comprise pipingsystems, or other devices well-known to the skilled person for thetransportation of gases, if needed under above or below (vacuum)atmospheric pressures.

Where it is spoken of “venturi scrubber” this can refer to either asingle venturi scrubber or a plurality of venturi scrubbers. Further,one or more venturi scrubbers can themselves comprises one or moreventuri tubes.

The invention pertains to urea finishing. This part of a urea productionprocess refers to the section where solid urea is obtained.

As an example, a schematic drawing of a plant having a finishing sectionin accordance with the invention is depicted in FIG. 1. For convenience,parts of the plant discussed below refer to the elements contained inFIG. 1. This does not imply that any plant built in accordance with theinvention needs to be in accordance with FIG. 1.

This finishing section, section (C) in FIG. 1, may be a prilling tower,granulation section, pelletizing section, or a section or equipmentbased on any other finishing technique. A granulation section may be afluidized bed-granulation, or a drum granulation, or a pan-granulation,or any other similar and known granulation device. The main function ofthis finishing section is to transfer a urea melt, as obtained from ureasynthesis, into a stream of solidified particles. These solidifiedparticles, usually called ‘prills’ or ‘granules’ is the main productstream from the urea plant. In any event, to transfer the urea from theliquid phase into the solid phase, the heat of crystallization has to beremoved. Moreover, usually some additional heat is removed from thesolidified urea particles, in order to cool them to a temperature thatis suitable for further processing and handling, including safe andcomfortable storage and transport of this final product. The resultingtotal removal of heat in the finishing section is usually done in twoways: (i) by evaporation of water. This water enters the finishingsection either as part of the urea melt, or is sprayed as liquid waterat an appropriate place in the finishing process; (ii) by cooling withair. Usually most of the crystallization/cooling heat is removed bycooling with air. The cooling air, by nature of the cooling process,leaves the finishing section at an increased temperature. Usually anamount of air equal to 3-30 kg of air per kg of final solidified productis applied, preferably 3-10 kg. This is the typical off-gas of thefinishing section of a urea production plant.

In the finishing section (C), the air comes into direct contact with theurea melt and with the solidified urea particles. This inadvertentlyleads to some contamination of the air with some urea dust, and ammonia.Depending on the nature of the finishing section (prilling/granulation,type of granulation, conditions selected in granulation), the amount ofdust present in the air may vary widely, values being in the range of0.05% to 10% by weight (with respect to the final product flow) havingbeen observed. For a finishing section based on granulation, the amountof dust more typically is in a range of from 2% to 8% by weight. Thispresence of dust in the off-gas usually makes a dust removal system (D)required, either for environmental or from economical considerations,before the air can be vented back into the atmosphere.

In the dust scrubbing section (D), dust scrubbing is usually done usinga circulating urea solution as a washing agent. On top of this alsofresh water scrubbing usually is applied. In the dust scrubbing sectionD a purge flow of urea solution is obtained. This purge flow usually hasa concentration of 10-60% (by wt.) of urea. In order to reprocess theurea present in this purge flow, the purge flow is returned to theevaporation section (B), where it is further concentrated and thenrecycled to the finishing section (C). Cleaned air is vented from thedust scrubbing into the atmosphere.

According to the invention, in one aspect, a method is provided for theremoval of urea dust from the off-gas of a finishing section of a ureaproduction plant. The method comprises subjecting the off-gas toquenching with water so as to produce quenched off-gas, and subjectingthe quenched off-gas to scrubbing using at least one venturi scrubber.

Quenching refers to adding water to the off-gas. This is generally doneby one or more quenchers, i.e. devices that serve to introduce waterinto the gas stream. This introduction will generally be done in such away that the water is well-dispersed into the gas. Preferably, the wateris introduced into the gas by spraying it into the gas flow line betweenthe finishing section and the dust scrubbing section. This can be doneby spraying liquid into a duct just preceding the dust scrubbingsection. It can also be a separate chamber or tower equipped with aspray system. Spray systems, suitable atomization nozzles, and the like,are known to the skilled person. Preferably, the liquid is sprayed insuch a way and consistency that liquid droplets are formed that are sosmall that the droplets evaporate quickly and a liquid saturation in thevapour near equilibrium is reached within a short time.

A quench section employing spray quenchers will preferably comprise (a)a section in which the gas to be quenched is cooled by the introduction(e.g. injection) and evaporation of water; (b) a dust collection basin,serving to collect dust stripped from the gas; (c) a sprayer system,mounted on a cylindrical portion and consisting of lances equipped withinjection nozzles, and a water supply system with pumps.

The off-gas (or “gaseous effluent”) coming from the finishing section,e.g. from a prilling tower of fluid bed granulator, is intended toinclude effluent streams that have liquid or solid particulate materialentrained therein, including vapours which may condense as the effluentstream is cooled.

In the quench zone, the gaseous effluent is cooled to a much lowertemperature, preferably below about 45° C. Many methods of cooling a hoteffluent gas flow are known to those skilled in the art.

A preferred method for use in the invention involves spraying a coolingliquid such as water, into the gas through nozzles. Without wishing tobe bound by theory, the inventors believe that spray-quenchingcontributes to the efficient removal of dust, by allowing water tointeract with dust particles.

This is an unexpected benefit of spray-quenching. In the art, notrelated to urea but, e.g., to flue gas, cooling of a gaseous effluenthas an effect in supersaturated systems. Therein, cooling the effluentcauses condensable vapours in the effluent stream to undergo phasetransition. Condensation of these vapours will naturally occur aroundparticles in the effluent stream which serve as nucleation points.Pre-cooling the effluent stream is, thus, useful for two reasons. Firstcondensable contaminants are transformed to the liquid phase and arethereby more easily removed from the effluent. Second, the nucleationprocess increases the size of pre-existing particles in the effluent,thereby making it easier to remove them.

The removal of the larger particles by quenching prevents the largerparticles from competing with the submicron particles as nucleationsites. As mentioned above, it is desirable that the submicron particlesincrease in size due to condensation so that they are easier to removefrom the effluent flow.

A problem with the gaseous effluent treated in the invention, i.e. theoff-gas of a finishing section of a urea plant, is that it is in asubsaturated state. As the sole condensable vapours, the off-gascontains a limited amount of water. As a result, it should be cooleddown to much lower values than achievable by quenching in order to havewater condensate as desired.

The fact that, by spray quenching, in the subsaturated urea finishingoff-gas an interaction with water is capable of contributing to theeffective removal of dust, thus is surprising. Without wishing to bebound by theory, the inventors believe that this effect is caused byevaporation of the sprayed water. This causes a lowering of thetemperature, and an increase of the amount of water in the gas-phase. Asa result, an interaction of water with submicron dust becomes possible.

A venturi scrubber is a known device, designed to effectively use theenergy from an inlet gas stream to atomize a liquid that is used toscrub the gas stream. A venturi scrubber consists of three sections: aconverging section, a throat section, and a diverging section. The inletgas stream enters the converging section and, as the area decreases, gasvelocity increases. Liquid is introduced either at the throat or at theentrance to the converging section.

The inlet gas, forced to move at extremely high velocities in the smallthroat section, shears the liquid from its walls, producing an enormousnumber of very tiny droplets. Particle and gas removal occur in thethroat section as the inlet gas stream mixes with a fog of tiny liquiddroplets. The inlet stream then exits through the diverging section,where it is forced to slow down. Venturis can be used to collect bothparticulate and gaseous pollutants, but they are more effective inremoving particles than gaseous pollutants.

Hence, a venturi scrubber by its nature is a suitable scrubbing devicefor the removal of urea dust from a gas stream. However, the use ofventuri scrubbers for this purpose, meets with limitations due to therelatively high operational costs associated with it. This particularlyrefers to the inevitable pressure drop occurring in a venturi scrubber,as a result of which a relatively high amount of energy input goes lost.The latter has adverse consequences for the energy consumption of theurea plant, and this is a concern both from an economical and anenvironmental perspective. Particularly the latter would mean that onetrades one drawback (air pollution) for another (energy consumption).

In accordance with the invention, the step of quenching the off-gas froma urea finishing section, before the gas is subjected to venturiscrubbing, has an unexpected dual effect.

On the one hand, the quenching of the off-gas from the urea finishingsection results in lowering the temperature of the off-gas entering theventuri scrubber. This lowering of temperature leads to a reduction ofthe gas volume and hence to a lowering of pressure drop. This, in turn,results in a higher percentage of energy conserved in the plant.

On the other hand, said quenching results in the presence of largeamounts of water droplets in the vapour phase, and hence to the thepresence of vapour in the airflow (off-gas) wherein urea dust ispresent. Based on theory, this would not have been expected to bringabout significant effects. In the art it is acknowledged thataerosol-particles (in the sub-micron and micron size range as is typicalfor urea dust) grow due to condensation of water on them fromsupersaturated gas that surrounds such particles. If the gas surroundingthe aerosols/particles is saturated or sub-saturated, but notsupersaturated, there is no growth or even negative growth of water fromthe wet surface of the aerosol-particle. As a result, the particleremains the same size or even evaporation from the surface of theparticle occurs. The general belief is that the degree ofsupersaturation (known as a factor S) needs to be larger than unity (1)to obtain condensation of water on aerosols, which is imperative toobtain growth of particles. Throughout the art on removing sub-microndust particles, it is acknowledged that effective removal requires anatmosphere in which water vapour is present in a supersaturated state.

Specifically in the art of urea finishing, such as in urea-granulationtechnology, it is recognized that it is impossible, in practice, toobtain a supersaturated gas-stream downstream of the finishing step.This can be explained with reference to the large amount of relativelydry air, and thus low presence of amounts of water, that are naturallypresent in the off-gas from urea finishing (e.g. from the granulator).In fact, the system initially (in the finishing section) starts fromalmost zero saturation, i.e., too strongly an undersaturated situationto reach a level of saturation, let alone supersaturation. Furthermoreconsidering the following:

-   -   the large amount of liquid droplets that are present acting as        seeds for condensation;    -   the short residence time in the quenching section;    -   the thermodynamic limitations (required energy for evaporation        of liquid water into vaporized water is not present);    -   the start of the system is strongly undersaturated;        in practice no supersaturated state can be reached in the        quenching section.

However, against the art-recognized beliefs, the inventor found that,surprisingly, a relatively large amount of condensation of water on themicron-size and sub-micron size urea particles takes place uponquenching. This leads to a significant growth of the micron-size andsub-micron-size particles. This growth of the sub-micron size particlesdue to condensation of water on them, leads to a significantly largerparticle size which makes the particles much easier to becollected/caught at acceptable pressure drops in the venturi sectiondownstream of the quenching section.

Overall, therefore, the method of the invention brings about a judiciouscombination of technical measures, that synergistically co-operate tomeet the aforementioned desires in the art. Particularly, the moreefficient removal of urea dust means that the venturi scrubber can beoperated at a lower pressure drop. Moreover, the quenching of theoff-gas before it enters the venturi scrubber, results in a smallervolume of gas, and therefore a lower pressure drop. Or, put otherwise,the desire to reduce the pressure drop over the venturi scrubber, whichis done by cooling the air entering the venturi, can be realized byanother (unexpected) effect of the quenching method used for saidcooling, viz. the urea particle growth, and hence more efficient removalthereof. Overall, the invention leads to very good collection/washingefficiencies for sub-micron urea particles, at a moderate pressure drop,allowing the use of smaller equipment, and consuming less power. Thelatter, on the one hand, is because of the lower pressure drop, on theother a lesser requirement for pressure drop because of the higherefficiency due to the unexpected effect of quenching.

The invention also pertains to the equipment for carrying out theabove-described method. This refers to a finishing equipment for a ureaplant. Therein a urea finishing device is present comprising theappropriate attributes to perform its function. These attributes areknown to the skilled person, and generally include an inlet for liquidurea, an inlet for cooling gas, a collector for solid urea (typically:urea particles, preferably granules), and an outlet for off-gas. Theoutlet for off-gas is in fluid communication (typically via a gas flowline) with the inlet of at least one venturi scrubber (the inlet beinginto the converging section). According to the invention, a quenchingsystem, preferably a spray quencher, is installed between the ureafinishing device and the venturi scrubber. It will be understood thatthe quenching system is installed in such a way that water sprayedtherefrom enters the gas stream that flows from the outlet of thefinishing section and the inlet of the venturi scrubber.

In a preferred embodiment, the dust removal system comprises a pluralityof venturi scrubbers, operated in parallel. Preferably, the dust removalsystem is so designed that these parallel venturi tubes can be operatedindependently of each other, i.e. the number of venturi tubes used atthe same time, can be adapted during the process as desired. A preferredsystem is that provided by Envirocare.

Envirocare scrubbers consist of a quenching section, downstream of whicha so-called MMV-section (micro-mist Venturi) is installed. TheMMV-section consists of multiple parallel venturis. In the MMV-sectionlarge quantities of liquid are sprayed in the throat of the venturisco-current with the gas-flow through single phase nozzles, creating aconsistent and adjustable liquid droplet-size, typically in a range offrom 50 μm to 700 μm. The liquid droplet size is one of the parametersthat can be used to control the efficiency of dust-removal

In the Venturi, intimate contact between particulate matter andwaterdroplets takes place. Multiple passages between particulate matterand water droplets takes place because initially the water droplets areaccelerated by the gas-flow (and thus have lower velocity than thegas-flow), while in the latter part of the venturi-tube, due toexpansion, the gas velocity decreases while the droplets are at velocityand maintain their velocity due to inertia (now liquid droplets have ahigher velocity than gas-flow).

Counter-currently with the gas-flow the so-called throat spray takesplace that controls the pressure drop over the venturi-section. In thisway fluctuations in gas-flow can be accommodated at more or lessconstant efficiency.

So, while in a standard venturi water-droplets (or, rather,water-fragments) are created by shear-forces, in the Envirocare concepta specific size (and shape) of water droplets is created. This ensures agood and efficient distribution of water and thus good washing. As aresult, while in a standard venturi-scrubber, the mixing of water isdepending of the quality of shear, the flow-patterns inside the throatand the diverging zone, in the Envirocare concept the mixing iscontrolled.

While a standard venturi scrubber's collection efficiency is stronglydepending on fluctuations in gas-flow (thus fluctuations in pressuredrop), the Envirocare scrubber controls the pressure drop by the throatspray.

The electricity consumption for a granulation section of a urea plant,utilizing a high efficiency venturi scrubber is estimated at 52 KWh/ton.Utilizing an Envirocare scrubber, with quenching, the electricityconsumption of the granulation section of a urea plant goes to 47kWh/ton.

Venturi scrubbing relies on the differential velocity between scrubbingdroplets and contaminant particles. The gaseous effluent and the spraydroplets both enter the inlet cone of the venturi at relatively lowvelocities. Differential velocities are achieved primarily as theparticles and droplets undergo acceleration through the throat of theventuri.

Normally, the contaminant particulates, being much smaller and havingmuch less mass, rapidly accelerate to attain the velocity of thesurrounding gas in a very short distance. On the other hand, thescrubbing liquid droplets are normally much larger and more massive, sothat it takes them much longer to attain the velocity of the gas stream.Typically, these droplets will not reach this ultimate velocity untilthe end of the throat or beyond the end of the throat. Since it is thevelocity differential which causes scrubbing, once the droplets andparticles reach the same velocity the number of interactions between thetwo will reduce to the point of insignificance, and no further scrubbingwill occur.

The scrubber contains a plurality of venturis (venturi tubes), housed inthe scrubber vessel. All of the venturis are substantially the same, andare of a similar design. The advantage of using multiple venturis isthat it permits a more compact overall design and reduces the size ofthe individual nozzles. Smaller nozzles are better able to produce thefine scrubbing droplets needed for efficiency. The number of venturitubes affects efficiency and pressure drop.

The scrubber design used in the invention is particularly well suited toretrofit existing pollution control equipment to improve scrubbingefficiency and lower operating costs. To retrofit an existing low energyimpingement scrubber, multiple venturis may be housed in the impingementchamber or in an extension to the chamber after one or more impingementplates.

A quenching section is disposed in the gas duct upstream of a MMVscrubbing tower and a scrubbing solution is provided at that section forquenching and cooling of the gas effluent coming from a Fluid BedGranulator (or other finishing section). The quench section performs thefunction of adiabatically humidifying or quenching the gas stream fromapproximately 100° C. to a temperature of about 50° C. using a scrubbersolution coming from the venturi scrubber vessel.

The invention also pertains to a urea plant comprising a finishingsection as described above. More particularly, the urea plant of theinvention, as illustrated in the example of FIG. 1, comprises asynthesis and recovery section (A); which is in fluid communication withan evaporation section (B). The evaporation section is in fluidcommunication with a finishing section (C), and has a gas flow line to acondensation section (E). The finishing section (C) has a gas flow lineto a dust scrubbing section (D). In accordance with the invention, thedust scrubbing section comprises at least one venturi scrubber (F), anda quenching system, preferably a spray-quencher (G). The quenchingsystem is installed between the finishing section (C) and the venturiscrubber (F), and is in fluid communication with the gas flow linebetween the finishing section (C) and the dust scrubbing section (D).Preferably, a plurality of venturi scrubbers is employed as outlinedabove. It will be understood that any desired number of venturis is influid communication (typically via a gas flow line) with the gas outletof the finishing section.

The invention is applicable to the construction of new urea plants(“grass root” plants) as well as in revamping existing urea plants.

It will be understood that a new plant according to the invention canjust be built in conformity with the above. In revamping existingplants, the invention pertains to a method of modifying an existing ureaplant, in such a way as to ensure that the plant has a dust scrubbingsection provided with at least one scrubber, and wherein a quenchingsystem is installed between the finishing section and the scrubber andthe scrubber is replaced or modified to a venturi scrubber. In anotherembodiment, in addition to one or more venturi scrubbers, an additionalacid scrubber can be used to improve the removal of ammonia. Thisscrubber is preferably placed downstream of the one or more venturiscrubbers.

The invention is not limited to any particular urea production process.

A frequently used process for the preparation of urea according to astripping process is the carbon dioxide stripping process as for exampledescribed in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27,1996, pp 333-350. In this process, the synthesis section followed by oneor more recovery sections. The synthesis section comprises a reactor, astripper, a condenser and a scrubber in which the operating pressure isin between 12 and 18 MPa and preferably in between 13 and 16 MPa. In thesynthesis section the urea solution leaving the urea reactor is fed to astripper in which a large amount of non-converted ammonia and carbondioxide is separated from the aqueous urea solution. Such a stripper canbe a shell and tube heat exchanger in which the urea solution is fed tothe top part at the tube side and a carbon dioxide feed to the synthesisis added to the bottom part of the stripper. At the shell side, steam isadded to heat the solution. The urea solution leaves the heat exchangerat the bottom part, while the vapour phase leaves the stripper at thetop part. The vapour leaving said stripper contains ammonia, carbondioxide and a small amount of water. Said vapour is condensed in afalling film type heat exchanger or a submerged type of condenser thatcan be a horizontal type or a vertical type. A horizontal type submergedheat exchanger is described in Ullmann's Encyclopedia of IndustrialChemistry, Vol. A27, 1996, pp 333-350. The heat released by theexothermic carbamate condensation reaction in said condenser is usuallyused to produce steam that is used in a downstream urea processingsection for heating and concentrating the urea solution. Since a certainliquid residence time is created in a submerged type condenser, a partof the urea reaction takes already place in said condenser. The formedsolution, containing condensed ammonia, carbon dioxide, water and ureatogether with the non-condensed ammonia, carbon dioxide and inert vapouris sent to the reactor. In the reactor the above mentioned reaction fromcarbamate to urea approaches the equilibrium. The ammonia to carbondioxide molar ratio in the urea solution leaving the reactor isgenerally in between 2.5 and 4 mol/mol. It is also possible that thecondenser and the reactor are combined in one piece of equipment. Anexample of this piece of equipment as described in Ullmann'sEncyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. Theformed urea solution leaving the urea reactor is supplied to thestripper and the inert vapour containing non-condensed ammonia andcarbon dioxide is sent to a scrubbing section operating at a similarpressure as the reactor. In that scrubbing section the ammonia andcarbon dioxide is scrubbed from the inert vapour. The formed carbamatesolution from the downstream recovery system is used as absorbent inthat scrubbing section. The urea solution leaving the stripper in thissynthesis section requires a urea concentration of at least 45% byweight and preferably at least 50% by weight to be treated in one singlerecovery system downstream the stripper. The recovery section comprisesa heater, a liquid/gas separator and a condenser. The pressure in thisrecovery section is between 200 to 600 kPa. In the heater of therecovery section the bulk of ammonia and carbon dioxide is separatedfrom the urea and water phase by heating the urea solution. Usuallysteam is used as heating agent. The urea and water phase, contains asmall amount of dissolved ammonia and carbon dioxide that leaves therecovery section and is sent to a downstream urea processing sectionwhere the urea solution is concentrated by evaporating the water fromsaid solution.

Other processes and plants include those that are based on technologysuch as the HEC process developed by Urea Casale, the ACES processdeveloped by Toyo Engineering Corporation and the process developed bySnamprogetti. All of these processes, and others, may be used precedingthe urea finishing method of the invention.

Urea finishing techniques, such as prilling and granulation, are knownto the skilled person. Reference is made to, e.g., Ullmann'sEncyclopedia of Industrial Chemistry, 2010, chapter 4.5. on urea.

The invention will be further illustrated hereinafter with reference tothe Example below. The Example is not intended to limit the invention.

Example

This Example refers to FIG. 2, which shows an exemplary dust scrubbingsystem of the present invention. An off-gas stream laden with entrainedurea dust-particles is generated by a finishing section 01. From thefinishing section 01, the off-gas stream 02 is delivered to thedust-scrubbing system though a duct 03.

The dust scrubbing system removes urea particles from the off-gas stream02 in two stages. In a first scrubbing stage, the so-called quenchingstage, the off-gas 02 flows though the quenching section 04, where themajority of large urea particles are removed from the off-gas, resultingin a partially scrubbed off-gas flow effluent as flow 05.

Furthermore in the quenching section 04, the off-gas 02 is cooled andmoistened with water. It is preferred that the gas in flow 05 is near tomoisture saturation.

For the purposes of cooling, saturating and scrubbing, a liquid flow 06is introduced through nozzles in the quenching section 04. The liquidflow 06 can either be a clean water flow or a urea-solution in water.The formed urea solution 07 is discharged from the quenching section.This urea solution 07 can either be discharged but can also be partlyrecycled to stream 06.

The following stage 08 is optional for the application in urea. Thestage 08 comprises of a washing stage in which condensed vapours andsize-enlarged micron and sub-micron particles for a part can be removedfrom the off-gas stream. For this purpose a washing liquid 09 isintroduced to the top-part of the washing stage, during percolating downthrough the washing stage 08, some collection of particles takes place.The formed urea solution 10 is discharged from this section 08. The ureasolution 10 will partly be recycled as washing liquid.

The off-gas stream flows to an MMV-section 11 (i.e. a Micro-MistVenturi). The off-gas flows through the venturis 11. Below each venturia liquid-spray nozzle co-currently sprays liquid-droplets in theventuris, the so-called MMV-spray 12. The liquid for the MMV-spray caneither be clean water or a urea solution in water.

Counter currently to the off-gas flow, the so-called throat-spray takesplace via nozzles inside the throat of the venturis. The throat-sprayliquid 13 can either be water or a solution of urea in water.

The wash-water from the MMV-stage containing the dissolved urea isdischarged as a liquid stream 14. Downstream of the MMV-section, ademister-section 15 is installed to capture droplets and/or wettedparticles. The demister is wetted by make-up water flow 16. The cleanedoff-gas 17 stream leaves the dust-scrubber.

1. A finishing equipment for a urea plant, said finishing equipment comprising a quenching system, one or more venturi scrubbers and a urea finishing device, wherein the urea finishing device comprises an inlet for liquid urea, an inlet for cooling gas, a collector for solid urea, and an outlet for off-gas, wherein said quenching system comprises an inlet that is connected with said outlet for off-gas of the urea finishing device and comprises an outlet for quenched off-gas, and wherein the one or more venturi scrubbers comprise(s) an inlet which is connected with said outlet for quenched off-gas.
 2. A finishing equipment according to claim 1, wherein the urea finishing device is a fluid bed granulation unit.
 3. A finishing equipment according to claim 1, wherein said venturi scrubber comprises a plurality of venturi tubes which are arranged in parallel.
 4. A finishing equipment according to claim 1, further comprising an acid scrubber for the removal of ammonia, which is arranged downstream of the one or more venturi scrubbers.
 5. A finishing equipment according to claim 1, wherein the one or more venturi scrubbers are of the micro mist venturi (MMV) type.
 6. A urea plant comprising a synthesis and recovery section, an evaporation section, a condensation section, a finishing section, a dust scrubbing section, and a quenching system, wherein said synthesis and recovery section is in fluid communication with said evaporation section, wherein said evaporation section is in fluid communication with said finishing section and has a gas flow line to the condensation section; and wherein said finishing section has a gas flow line to said dust scrubbing section, wherein the dust scrubbing section comprises at least one venturi scrubber, wherein said finishing section comprises a urea finishing device comprising an inlet for liquid urea, an inlet for cooling gas, a collector for solid urea, and an outlet for off-gas, wherein said quenching system comprises an inlet that is connected with said outlet for off-gas of the urea finishing device and comprises an outlet for quenched off-gas, and wherein the at least venturi scrubber comprises an inlet which is connected with said outlet for quenched off-gas.
 7. A method of modifying an existing urea plant, said plant comprising a synthesis and recovery section (A), an evaporation section (B), a finishing section (C), a dust scrubbing section (D) and a condensation section (E), wherein said synthesis and recovery section (A) is in fluid communication with said evaporation section (B), wherein said evaporation is in fluid communication with said finishing section (C) and has a gas flow line to a condensation section (E); wherein said finishing section (C) has a gas flow line to a dust scrubbing section (D), wherein the dust scrubbing section (D) comprises at least one venturi scrubber (F), wherein the method comprises installing a quenching system (G) between the finishing section (C) and the at least one venturi scrubber (F), wherein said quenching system is in fluid communication with a gas flow line from the finishing section (C) and with a gas flow line to venturi scrubber (F) comprised in the dust scrubbing section (D).
 8. A method according to claim 7, wherein the method comprises installing one or more venturi scrubbers in parallel to the already present venturi scrubber.
 9. A method according to claim 7, wherein the plant further comprises an acid scrubber for the removal of ammonia downstream of the dust scrubbing section.
 10. A method according to claim 7, wherein the venturi scrubber is of the micro mist venturi (MMV) type.
 11. The finishing equipment according to claim 1, wherein the at least one venturi scrubber is configured for subjecting the quenched off-gas to scrubbing to give scrubbed gas, wherein said venturi scrubber comprises a throat, a converging section, and an entrance to said converging section, wherein said venturi scrubber is furthermore configured for adding a scrubbing liquid to the venturi scrubber at said throat or at said entrance such that said added scrubbing liquid enters the quenched off-gas wherein said quenching system is configured for subjecting the off-gas to quenching with water optionally in the form of an aqueous solution so as to produce quenched off-gas, wherein the quenching system is arranged upstream of the venturi scrubber such that the quenching takes place before the off-gas enters the venturi scrubber, and wherein the finishing equipment further comprises a washing section comprising an inlet for a washing liquid and configured for percolation of the washing liquid down through said washing section thereby collecting particles from said quenched off-gas stream.
 12. The urea plant according to claim 6, wherein the at least one venturi scrubber is configured for subjecting the quenched off-gas to scrubbing to give scrubbed gas, wherein said venturi scrubber comprises a throat, a converging section, and an entrance to said converging section, wherein said venturi scrubber is furthermore configured for adding a scrubbing liquid to the venturi scrubber at said throat or at said entrance such that said added scrubbing liquid enters the quenched off-gas wherein said quenching system is configured for subjecting the off-gas to quenching with water optionally in the form of an aqueous solution so as to produce quenched off-gas, wherein the quenching system is arranged upstream of the venturi scrubber such that the quenching takes place before the off-gas enters the venturi scrubber, and wherein the finishing equipment further comprises a washing section comprising an inlet for a washing liquid and configured for percolation of the washing liquid down through said washing section thereby collecting particles from said quenched off-gas stream.
 13. The method of modifying an existing urea plant according to claim 8, wherein the at least one venturi scrubber is configured for subjecting the quenched off-gas to scrubbing to give scrubbed gas, wherein said venturi scrubber comprises a throat, a converging section, and an entrance to said converging section, wherein said venturi scrubber is furthermore configured for adding a scrubbing liquid to the venturi scrubber at said throat or at said entrance such that said added scrubbing liquid enters the quenched off-gas wherein said quenching system is configured for subjecting the off-gas to quenching with water optionally in the form of an aqueous solution so as to produce quenched off-gas, wherein the quenching system is arranged upstream of the venturi scrubber such that the quenching takes place before the off-gas enters the venturi scrubber, and wherein the finishing equipment further comprises a washing section comprising an inlet for a washing liquid and configured for percolation of the washing liquid down through said washing section thereby collecting particles from said quenched off-gas stream. 